It is known in the art that silver ions can form water-soluble complexes with numerous inorganic and organic compounds. Specific silver complexes include the AgCl.sub.2.sup.-, AgCl.sub.4.sup.-3, AgBr.sub.3.sup.-2, AgBr.sub.5.sup.-4, Ag.sub.2 Br.sup.+, AgI.sub.2.sup.-, AgI.sub.4.sup.-3, AgCl.sub.3 Br.sup.-3, Ag.sub.3 I.sup.-2, and AgClBr.sub.3.sup.-3 complexes. These complexes are referred to as water-soluble complexes of silver halide complexed with excess halide. Other complexing agents for silver halide include ammonia and the amines, monovalent anions such as cyanide, thiocyanate and the like, polyvalent anions such as thiosulfate, sulfite and the like, and organics, such as acetic acid, glycine, thiourea and the like. It is believed that the foregoing complexes can participate in the growth of the silver halide crystals during physical ripening of the emulsion and influence the kinetics of development. The complexes, including the aforementioned, are known to the art and are set forth on pages 8 and 9 of Mees, The Theory of the Photographic Process, 3rd Ed., MacMillan Co., New York. U.S. Pat. Nos. 3,883,355, issued May 13, 1975; 3,941,600, issued Mar. 2, 1976; and 4,153,462, issued May 8, 1979 employ the above-described water-soluble complexes of silver halide in forming photosensitive silver halide emulsion layers.
U.S. Pat. No. 4,153,462 is directed to a method for forming a silver halide photographic emulsion which comprises forming a solution of a water-soluble complex of silver halide and breaking the complex by dilution to provide for the precipitation of the silver halide crystals. The dilution medium contains an agent for modulating the crystallization of the grains. By the selection of the particular modulating agent the size, habit, composition, sensitivity and other properties of the silver halide grains can be affected.
The silver halide complex is preferably added rapidly to the diluting medium and decomplexation and grain formation is substantially instantaneous.
Subsequent to grain formation, the crystals may be washed, treated as desired, for example, by the addition of sensitizing agents and then dispersed in a suitable binder to provide a photographic silver halide emulsion.
U.S. Pat. No. 3,941,600 is directed to a method for forming a photographic silver halide emulsion layer which comprises imbibing a relatively thin layer of a permeable gel substrate with a solution of a water-soluble silver halide complex and, subsequent to imbibing said substrate, breaking the complex to form water-insoluble silver halide grains within the substrate. Decomplexation is achieved by dilution of the water-soluble complex which results in the depletion of the concentration of the halide ion. Excess complex salts remaining associated with the substrate are easily removed by washing.
U.S. Pat. No. 3,883,355 is directed to a method for forming a photographic silver halide emulsion layer which comprises the steps of mixing an aqueous solution of a water-soluble complex of silver halide with a polymeric binder material, forming a layer of the thus-formed mixture and then breaking the complex of the silver halide complex disposed in said layer to form silver halide crystals therein. Decomplexation is preferably achieved by treating the layer with a material to reduce the halide ion concentration, for example, by contacting the layer with sufficient water to accomplish decomplexation by dilution.
Of the above-mentioned methods for forming silver halide emulsion layers, one of the most desirable is the method set forth in U.S. Pat. No. b 3,941,600 wherein the water-soluble silver halide complex is applied to an already formed permeable substrate wherein the silver halide crystals are then formed in said substrate.
This method does not require the bulk precipitation and washing of silver halide in large vessels or other such operations which would be time-consuming or have extensive energy consumption requirements.
However, it has been found that the thickness of the substrate can have an effect on the size of the silver halide grains formed therein. It has also been found that when larger grains are formed they are formed relatively near the surface of the substrate and are subject to being removed by washing or abrasion of the substrate during the handling of the coated substrate.